This proposal is based upon the general hypothesis that the trophoblast, which forms the primary barrier between mother and fetus, employs defense mechanisms against the afferent and efferent arms of the mother's immune system in order to protect the semi-allogeneic fetal-placental unit from destruction by maternal immune responses. In most species the mechanisms by which the conceptus evades maternal immune recognition dominate so strongly that it is difficult to test the vulnerability of trophoblast cells to immune attack. We propose to study horses, donkeys, and their interspecies hybrids because the strong maternal anti-fetal immune responses mounted during pregnancies within and between these species permits investigations that cannot be easily addressed in any other system. These include mechanistic studies of 1) antigen presentation and maternal sensitization; 2) the means by which Major Histocompatibility Complex (MHC) class I (+) trophoblasts evade immune destruction; and 3) the cellular pathways that result in trophoblast cell death. We propose to study three aspects of maternal anti-fetal immune responses: 1. How and where does the mother become sensitized to paternal MHC class I antigens expressed on trophoblast cells? First, we would explore the site of antigen presentation by determining whether invasive trophoblast cells reach the lymph nodes draining the uterus via the uterine lymphatics. Second, we would investigate the antigenic specificity of uterine lymphocytes by determining the T cell receptor (TCR) variable gene usage of T cells which accumulate around the invasive endometrial cup trophoblasts and comparing this to TCR variable gene usage in other lymphoid compartments. Finally, we would test the hypothesis that the antigenic stimulus of pregnancy evokes an immune response that is qualitatively different from the induced by classical transplantation. 2. What prevents the invasive MHC class I antigen (+) trophoblast cells from being destroyed by maternal immune responses? We would test the susceptibility of MHC class I (+) trophoblast cells to killing by maternal cytotoxic lymphocytes and antibody and complement using in vitro assays. We would test the hypothesis that uterine growth factors and hormones modulate maternal immune effector mechanisms in the uterus. 3. What mechanisms bring about the destruction of the MHC class I (-) invasive trophoblast cells of the endometrial cups? We would test the hypothesis that indirect presentation of trophoblast-specific antigens by MHC class II molecules of endometrial glandular epithelial cells or uterine macrophages results in the destruction of the endometrial cups through activation of T cells. These comparative studies of pregnancy and histocompatibility should provide unique insights into immunoregulatory mechanisms operating in the uterus. Additionally, they may reveal important links between the behavior of the fetal allograft and its maternal host and the success and failure of tumors and clinical organ transplants.